Reducing an elephant's power

Question

I am planning an encounter for a story driven campaign. A 4 player second level party encounters an escaped war elephant, who is making a farmer's life miserable and wrecking his crops. A war elephant is supposed to be a terrifying opponent. A squadron of them is capable of smashing a phalanx of soldiers. In keeping with this, the 5e stats for a regular elephant have it worth 1100 xp. A deadly encounter for this party would be 800 XP, significantly less than the elephant. Although I have a couple of non-combat solutions prepared, I want a combat solution to be on the table; I want a combat solution to be risky but doable. I like the description of "hard" in the DMG; I want the players to worry that this could go badly, but also believe that they will probably win.

Hard. A hard encounter could go badly for the adventurers. Weaker characters might get taken out of the fight, and there's a slim chance that one or more characters might die.

As written in the monster manual, the elephant does an average of 39 damage if it hits with both its gore and its stomp; enough to turn a second level fighter into giblets. It also has enough HP that the party would take several rounds to kill it, even if it rolls nothing but 1s.

What can I do to nerf this elephant into a more doable encounter? How do I know that this encounter is actually level appropriate?

How can I signal these nerfs to the players? I want them to know what choice they are making, before they make it.

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I have a few ideas for solutions banging around my head, but I don't know that they will work. If I over-nerf the elephant, the combat is trivial. If I under-nerf it, I kill the players.

Solution 1: A barbed spear is hanging out of the elephant's side and pus is seeping from the wound. Mechanically the elephant has fewer hit points, but he can still murder any PC in a single round. This doesn't do everything I want.

Solution 2: The elephant is tangled in its armor, and has its move speed reduced to 15 feet. This prevents it from using its "trampling charge" ability. The average damage would fall from 39 to 18, still a very serious amount. It can down any party member in one hit if it rolls decently, and kill the squishy ones. This also means that a clever party member with a bow can stand back and snipe the thing indefinitely.

Solution 3: The elephant is severely dehydrated. It has the level of exhaustion that gives it disadvantage on all rolls.

I want to know if these solutions adjust a elephant to the proper difficulty level. I will also accept answers with other solutions.

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If necessary I will accept answers from experience, but I would very much appreciate written guidelines. This situation will probably come up again, and I would like to be able to answer it for myself.

Answer 1

This is a general answer, not a specific one, but it applies to your problem:

Run the encounter ahead of time.

Numbers will only take you so far, as they cannot take into account every possible variable, or deal with things like player experience and tactics, the vagaries of environment or battlefield control spells. As DM, you should have access to their character sheets. You should have a likely idea of where the encounter will take place. Set out minis, sketch out a battlefield, and pretend that the elephant is the PC and you the DM are running the characters as NPCs trying to kill it. This will help you get into the mindset of the players, who are racking their brains and checking their sheets for resources they can bring to bear and tactics they can use.

I can practically guarantee that the elephant will not fare nearly so well as you fear. The action economy of multiple characters against one is pretty powerful, and their creative use even more so.

At the very least, you will learn things that were pivotal elements, like the importance of cover or tight spaces. If you feel it necessary, increase or decrease those elements: provide more opportunities for cover on your battlefield, or obstacles such as a wide deep ditch that the elephant cannot pass. If a character dies, analyze why. What would have given them an opportunity for survival: "If only I could have had a safe place from which to fire my arrows!" -- put in a huge old tree. And so forth.

Whatever you do, don't just sit there and try to run the combat by comparing vanilla hit and damage rolls. Your players will likely be creative to a degree that invalidates that method.

Should you do this with every encounter? Heck no! You have better things to do with your time. But doing it several times as you get used to a system or to DMing in general can give you a much better feel for relative strengths.

Answer 2

No official answer

The Monster Manual writes about Modifying a Monster on page 273, but no specific rules about nerfing.
However, you can calculate the CR of a creature identical to the Elephant, and the changes your solutions would mean.

Elephant base CR

The Monster Manual lists it as 4, but the math does not add up. It should be 3.

Defensive CR

Based on HP (76) it should be CR1.
The Armor Class (12) is lower than expected for CR1, but not so much that it decreases the value, so it stays CR1.

Offensive CR

The calculation is more complicated* than just adding together the average damages of its different attacks. As the Elephant has no multiattack, triggering Stomp depends on many things.

30 is the expected damage*, this means CR4.
The Elephant's attack bonus is the only thing that is truly spectacular for a monster of its CR, increasing the offensive CR to 5.

Final CR

The average of 5 and 1 is 3, that should be the CR of the Elephant. I assume someone in the MM team just added together Stomp and Gore to get 4.

The DMG says if the monster relies on saving throws, the CR should be calculated with the save DC column. The Stomp only triggers after the save is falied, and looking up its DC gives a CR of 0. So calculating this way, the final CR would be even lower.

Your Solutions

All of your proposals work fine, decreasing the CR further.

1. The elephant already has the hit points of a CR1 creature, and the AC of CR0. Further reduction of HP might actually make the encounter too easy
2. Taking away Stomp would reduce the CR to 2.5. If a squishy character lets something without ranged attacks and a speed of 15 even attack it, he deserves to die
3. It will not be a fight, but a butchering of nearly defenseless animal (if you mean exhaustion 3, it does not matter much below that). Constant disadvantage is about equivalent to -4 for attacks, the Elephant is now CR2, losing its only forte.

Conclusion

Your party should be able to win without any changes, if the squishies do not charge into melee.

If you still have doubts, you can use any of your proposed solutions to assure an easy victory.

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*For Stomp to trigger, the target still has to fail a quite easy saving throw, and be hit again. We have to calculate the likelyhood of this, to get the expeced damage of the Elephant.
Take a Wizard as worst case scenario, Str save -1, AC 14 (Mage Armor + 12 Dex). After a successful Gore, the chance of him failing the DC12 save is 60%, and 75% chance of being hit. This is 45% to trigger Stomp.
For a Fighter, the Str save would be around +6, AC 18 (Chain Mail + Shield). Failing the save has only 25%, and 55% of the Elephant hitting her again. Only 13.75% chance to trigger Stomp.

Let us calculate with a very generous 50% chance of Stomp triggering, this puts the expected damage 19 + 22 / 2 = 30.

Answer 3

Working without numbers

If you are concerned with particular elements of a monster's fighting "style", changing the environment of the fight can work wonders. If they fight the elephant in a featureless 70x70 ft. room, it will probably flatten them. There is not enough space to keep a distance or outmaneuver the animal's speed. If they fight it in a dense forest, suddenly everything is different. There might not be enough space for a Huge creature to charge, ranged fighters can position themselves farther away and there are plenty of opportunities to hide against the beast's not stellar perception.

You should also take into account possible clever tactics the PC-s might (or might not) employ. Fighting a well-known, lone opponent presents a host a possibilities. You should think about some of those yourself and make sure that the circumstances of the battle do not prohibit them if you want to give your players an easier time. For example hit&run or guerilla style tactics will not have much effectiveness on a wide open plain or against an opponent that can regenerate fast.

Working with numbers

If you still wish to fiddle with numbers, invent new numbers first (HP, hit chance, ...), to fit your preferences. You can then rationalize these in any way you want. I cannot help with determining what would sound reasonable given your story and world, but you can literally invent anything. Go as wild as you want.

Checking a solution

Working with vague things like tactics and environmental factors is not easy if you want an exact measure of difficulty. Try playing out the fight with different parameters for 3-4 rounds. You can use featureless plane vs. head-on charge as a benchmark and work from there.

If you really want numbers, recalculate the CR of the monster with the possibly new stats or other factors added in. Since you are evaluating a concrete situation, rather than a monster "in the void", they should even be more accurate.

Note: Beware of CR numbers in the MM, as they often do not even follow their own guidelines, as presented in the DMG (starting on p.273). The CR of the elephant should be 3, for example (defensive CR 1, offensive CR 5).